replace "conventions" with "high-level overview"

Also bring in material from the librustc README.md

Author: nikomatsakis

src/SUMMARY.md

@@ -4,7 +4,7 @@
 - [How to build the compiler and run what you built](./chap-010-how-to-build-and-run.md)
 - [Using the compiler testing framework](./chap-020-running-tests.md)
 - [Walkthrough: a typical contribution](./chap-030-walkthrough.md)
-- [Conventions used in the compiler](./chap-040-compiler-conventions.md)
+- [High-level overview of the compiler source](./chap-040-high-level-overview.md)
 - [The parser](./chap-050-the-parser.md)
 - [Macro expansion](./chap-060-macro-expansion.md)
 - [Name resolution](./chap-070-name-resolution.md)

src/chap-040-compiler-conventions.md

@@ -0,0 +1,141 @@
+# High-level overview of the compiler source
+
+## Crate structure
+
+The main Rust repository consists of a `src` directory, under which
+there live many crates. These crates contain the sources for the
+standard library and the compiler.  This document, of course, focuses
+on the latter.
+
+Rustc consists of a number of crates, including `syntax`,
+`rustc`, `rustc_back`, `rustc_trans`, `rustc_driver`, and
+many more. The source for each crate can be found in a directory
+like `src/libXXX`, where `XXX` is the crate name.
+
+(NB. The names and divisions of these crates are not set in
+stone and may change over time -- for the time being, we tend towards
+a finer-grained division to help with compilation time, though as
+incremental improves that may change.)
+
+The dependency structure of these crates is roughly a diamond:
+
+```
+                  rustc_driver
+                /      |       \
+              /        |         \
+            /          |           \
+          /            v             \
+rustc_trans    rustc_borrowck   ...  rustc_metadata
+          \            |            /
+            \          |          /
+              \        |        /
+                \      v      /
+                    rustc
+                       |
+                       v
+                    syntax
+                    /    \
+                  /       \
+           syntax_pos  syntax_ext
+```                    
+
+The `rustc_driver` crate, at the top of this lattice, is effectively
+the "main" function for the rust compiler. It doesn't have much "real
+code", but instead ties together all of the code defined in the other
+crates and defines the overall flow of execution. (As we transition
+more and more to the [query model](ty/maps/README.md), however, the
+"flow" of compilation is becoming less centrally defined.)
+
+At the other extreme, the `rustc` crate defines the common and
+pervasive data structures that all the rest of the compiler uses
+(e.g., how to represent types, traits, and the program itself). It
+also contains some amount of the compiler itself, although that is
+relatively limited.
+
+Finally, all the crates in the bulge in the middle define the bulk of
+the compiler -- they all depend on `rustc`, so that they can make use
+of the various types defined there, and they export public routines
+that `rustc_driver` will invoke as needed (more and more, what these
+crates export are "query definitions", but those are covered later
+on).
+
+Below `rustc` lie various crates that make up the parser and error
+reporting mechanism. For historical reasons, these crates do not have
+the `rustc_` prefix, but they are really just as much an internal part
+of the compiler and not intended to be stable (though they do wind up
+getting used by some crates in the wild; a practice we hope to
+gradually phase out).
+
+Each crate has a `README.md` file that describes, at a high-level,
+what it contains, and tries to give some kind of explanation (some
+better than others).
+
+## The main stages of compilation
+
+The Rust compiler is in a bit of transition right now. It used to be a
+purely "pass-based" compiler, where we ran a number of passes over the
+entire program, and each did a particular check of transformation. We
+are gradually replacing this pass-based code with an alternative setup
+based on on-demand **queries**. In the query-model, we work backwards,
+executing a *query* that expresses our ultimate goal (e.g., "compile
+this crate"). This query in turn may make other queries (e.g., "get me
+a list of all modules in the crate"). Those queries make other queries
+that ultimately bottom out in the base operations, like parsing the
+input, running the type-checker, and so forth. This on-demand model
+permits us to do exciting things like only do the minimal amount of
+work needed to type-check a single function. It also helps with
+incremental compilation. (For details on defining queries, check out
+`src/librustc/ty/maps/README.md`.)
+
+Regardless of the general setup, the basic operations that the
+compiler must perform are the same. The only thing that changes is
+whether these operations are invoked front-to-back, or on demand.  In
+order to compile a Rust crate, these are the general steps that we
+take:
+
+1. **Parsing input**
+    - this processes the `.rs` files and produces the AST ("abstract syntax tree")
+    - the AST is defined in `syntax/ast.rs`. It is intended to match the lexical
+      syntax of the Rust language quite closely.
+2. **Name resolution, macro expansion, and configuration**
+    - once parsing is complete, we process the AST recursively, resolving paths
+      and expanding macros. This same process also processes `#[cfg]` nodes, and hence
+      may strip things out of the AST as well.
+3. **Lowering to HIR**
+    - Once name resolution completes, we convert the AST into the HIR,
+      or "high-level IR". The HIR is defined in `src/librustc/hir/`; that module also includes
+      the lowering code.
+    - The HIR is a lightly desugared variant of the AST. It is more processed than the
+      AST and more suitable for the analyses that follow. It is **not** required to match
+      the syntax of the Rust language.
+    - As a simple example, in the **AST**, we preserve the parentheses
+      that the user wrote, so `((1 + 2) + 3)` and `1 + 2 + 3` parse
+      into distinct trees, even though they are equivalent. In the
+      HIR, however, parentheses nodes are removed, and those two
+      expressions are represented in the same way.
+3. **Type-checking and subsequent analyses**
+    - An important step in processing the HIR is to perform type
+      checking. This process assigns types to every HIR expression,
+      for example, and also is responsible for resolving some
+      "type-dependent" paths, such as field accesses (`x.f` -- we
+      can't know what field `f` is being accessed until we know the
+      type of `x`) and associated type references (`T::Item` -- we
+      can't know what type `Item` is until we know what `T` is).
+    - Type checking creates "side-tables" (`TypeckTables`) that include
+      the types of expressions, the way to resolve methods, and so forth.
+    - After type-checking, we can do other analyses, such as privacy checking.
+4. **Lowering to MIR and post-processing**
+    - Once type-checking is done, we can lower the HIR into MIR ("middle IR"), which
+      is a **very** desugared version of Rust, well suited to the borrowck but also
+      certain high-level optimizations. 
+5. **Translation to LLVM and LLVM optimizations**
+    - From MIR, we can produce LLVM IR.
+    - LLVM then runs its various optimizations, which produces a number of `.o` files
+      (one for each "codegen unit").
+6. **Linking**
+    - Finally, those `.o` files are linked together.
+
+
+
+
+The first thing you may wonder if